Modified power head power draw

[Mutagen]

Richard,

Just thought I'd add some info about modifying pumps. I'm sure you already checked out potential problems but I didn't see any comments in the article.

Pumps and motors are designed as a unit with the motor chosen such that its power output matches the power requirements of the pump to which it's coupled. The modification you write about is essentially changing out a pump but still using the same motor. There is a very good chance the propeller is moving much more water than the original pump and is therefore drawing considerably more power. If this is so, the motor is likely being over driven and may be producing more heat than its designed to handle. I assume the change out has been tested for some time and no problems have been observed so no mention of said possibility has been made. However, other hobbyists may take the idea further and modify other power heads with a smaller motor safety factor. This could result in a failure of the motor and most of us really don't want to fry a power head in our reefs. Hence I present the warning here.

Also, since the modified pump is in all likely hood moving much more water than prior to the modification some of the effects you observed were likely due to the increase in overall water movement and not due only to a change in the flow pattern. Not to say this is bad, but it is likely happening. If no long term issues occur with the motor, it seems like a nice little power head upgrade.

Finally, it is also fairly easy to add a nozzle on the end of a power head to change the flow pattern from tight to wide. The simplest is just to add a larger diameter pipe to the end of the power head. Even better is a larger diameter pipe that is open on the sides so that the forward motion of the higher velocity tight stream pulls in water from the sides. This converts some of the pressure energy of the pump into flow energy in the form of more water moving at a lower velocity. Of course, this nozzle arrangement will not change the actual amount of water going through the power head so the pump will not be over driven. The propeller modification will almost certainly make the most total flow.

It would be interesting to see the power draw of the pump before and after the modification.

Anyway, thanks for the article and the info it contains,

Tracy Gray

 

[Anonymous]

hi.
Several manufacturers are experienced in propeller pump. However, with maybe one or two exceptions, all of them are for commerical, hi-flow (1000 gal/min) application like irrigation.

Even with propeller pumps suitable for the hobby (50gal/min-300gal/min), the price is still high ($400 to $1000). The real advantage is its efficiency in low pressure (<5psi or 10ftH2O). For example, what centrifugal pump do you know can do 80gal/min with only ~100W power consumption? By doing the modification, you can save a lot of money but using a regular powerhead, and get the kit, instead of getting the commerical product. DIY for this is easy, and save you money.

The jet pump application was mentioned in the above message. Again, this type of pump is usually used in well and irrigation application. The application of jet pump in the hobby was looked at several times in the past by individuals, but nothing has come out of it commerically.

 

[rharker]

Pumps and motors are designed as a unit with the motor chosen such that its power output matches the power requirements of the pump to which it's coupled. The modification you write about is essentially changing out a pump but still using the same motor. There is a very good chance the propeller is moving much more water than the original pump and is therefore drawing considerably more power. If this is so, the motor is likely being over driven and may be producing more heat than its designed to handle. Tracy Gray

Tracy:

You raise a good point that the article did not address. Were one to mount a larger prop on the motor, there might be the risk that the current drawn would be excessive, but that doesn't seem to be the case with the prop supplied with the kit. The unmodified pump draws a half amp and the modified pump with prop draws a little more. According to my powermeter there is a difference of 4 watts, or about 10%.

To put that into perspective, the Otto 2000 draws the same current as the modified Little Giant but runs much warmer. If anything I would say that the pond pumps have a higher margin of safety than a typical powerhead. They are also thermally protected.

Richard Harker

 

[mkirda]

You raise a good point that the article did not address. Were one to mount a larger prop on the motor, there might be the risk that the current drawn would be excessive, but that doesn't seem to be the case with the prop supplied with the kit. The unmodified pump draws a half amp and the modified pump with prop draws a little more. According to my powermeter there is a difference of 4 watts, or about 10%.

Richard,

When you were sent the beta unit, did jc send you the larger prop as well (the non-yellow one)? Have you/could you test the PE-1 with it and compare it?

Having run this prop on the PE-1 for close to six months now, it have to say that it works great for it's current application, in my current tank, and that the powerhead does not seem to be affected adversely. I do suspect, however, that it does draw more power... I am curious how much more.

Note to everyone: I don't recommend this prop over the yellow one jc provides. The yellow one has a much greater amount of flow, and is what everyone should use. I will be switching to the yellow one as soon as I move into our new house and get my tank back into my tank, rather than my Rubbermaid horse trough...
(Can you tell I'm trying to head off yet more e-mail? LOL.)

Regards.
Mike Kirda

 

[rharker]

When you were sent the beta unit, did jc send you the larger prop as well (the non-yellow one)? Have you/could you test the PE-1 with it and compare it?

No, I've just been using the small prop, so I haven't tested the larger prop. I'll try to do that, but as you point out, the smaller prop creates quite a bit of current and I'm not quite sure anyone really needs the larger prop.

I'll let everyone know what I've checked it out.

Richard Harker

 

[mkirda]

I'm not quite sure anyone really needs the larger prop.

I'll let everyone know what I've checked it out.

Richard Harker

No, just me! I'm the only one... LOL.

I've explained in a RC post, but the yellow prop has too much force when pointed straight up, hence my switch to the larger prop. I just want to avoid any potential splash or spray onto my lights.

[collective 'Ahhh' sound from all who read this post...]

Thanks, Richard. I am just curious how much more draw it has. It probably isn't a pump-killer though, otherwise I suspect it would have died by now...

Regards.
Mike Kirda

 

[Mutagen]

Richard,

Thanks for the power data. I agree 10% extra should be no problem at all. Given the problems some people have had with "certain" brand power heads I really didn't want to depend on the thermal protection in the motor! Since I suspect the modified pump is moving at least 50% more water this seems like a great upgrade. Now maybe a power head manufacturer will finally build an axial flow pump. (As seven ephors points out, this type of pump is the properly specified pump for the application)

One other pertinent comment, axial flow pumps perform poorly if they are throttled even to fairly a small degree. Their power draw will go up quickly and the flow rate drops very quickly as well. In other words, if you try to direct the flow from the modified pump, don't try to hook it up to anything but the most free flowing of channels.

Seven,

I have used the jet pump idea myself rather succesfully in a small tank so I know this works, but I still prefer the propeller modification since it bypasses the less efficient desgin of the typical power head centrifugal pump.

For what its worth, axial flow pumps are not (theoretically) really that much more efficient than well made centrifugal pumps, but there is an important difference. When a pump manufacturer specifies a pump's efficiency it reflects the degree to which the electrical energy provided to the pump is converted to fluid energy. The fluid energy takes the form of kinetic energy and pressure energy. Axial flow pumps are typically very low pressure pumps so the flow rate leaving the pump is high since almost the electrical energy is used to generate flow. A centrifugal pump converts more of the electrical energy into pressure energy and less into flow energy, so for the same power input the flow rate is lower. However, once the lower volume higher pressure water from the centrifugal pump enters the tank, the pressure energy is converted into heat energy through friction and also into some kinitic energy through the action of the high velocity water on the low velocity water around it. If the conversion of pressure energy into flow energy were efficient, little heat would be generated, and the total movement of water in the tank would actually be fairly similar for both pumps. So the losses are in two places, the inefficiency inherent in converting pressure energy into kinetic energy in the tank itself and also in the fact that the typical powerhead is not manufactured such that it approaches the efficiency of an axial pump. Given the simplefied designs of most power head pump casings, I suspect they are between 60 and 70% efficient. The modified axial flow pump is probably more like 80-85% efficient even if the flow is not particularly well directed.

Actually, it sounds like maybe this is something you already knew, but its worth stating for those less familiar with the various pump designs.

Tracy

 

[Anonymous]

hi.
To illustrate your comment, consider that a 760 gal/hr powerhead using 50W of electricity. With a 1/2 inch opening, it will have about 1 (one) watt of kinetic energy (mv^2 /2). The pressure energy varies depends on the load. Assume that the head pressure is 5 feet (~2psi), and the flow rate is 750 gal/hr, the pressure energy is ~11 watt (PV/t). So the efficiency is less than 25% (K+P /E).

As you can see (if I use the correct formular), the ratio of pressure energy to kinetic energy is more than 10:1. For propeller pump with similar electricity consumption, the flow rate is ~2000 gal/hr. Typical output opening is at least 2 inch, so the kinetic energy is still about 1 watt, but the pressure energy at one feet pressure (assume flow rate is 1500 gal/hr) is almost 5 watt. But it seems to say that the efficiency is only slightly more than 10%.

So did I use the right reasoning/formula for my calculation, or I must match the pressure/flow rate pair to get the total energy?

 

[Mutagen]

Seven ephors,

To be honest, I had not actually carried out the calculations to check energy consumption vs the ratings on power heads. I was judging the pump efficiencies against typical industrial applications where well made centrifugal pumps have efficiencies from 75% to 82-83%. So when i read the results of your calc's at 25% efficiency I was astounded. I mean, could the little power heads actually be THAT bad? So I also checked the calc's using the same basic assumptions as you made. I am assuming you are taking performance data from a manufacturer's rated performance for a specific power head. I did get different results in one case, while the other results are verified almost perfectly. This leads me to believe that in the once instance, you missed a term or unit conversion. Here is what I got:

Basis:
750 GPH
5 ft of head at pump discharge
1/2 inch diameter opening at pump discharge
50 watts of power being used

15.3 watts power to provide the kinetic energy
11.8 watts power to provide the pressure component of total energy
27.1 watts converted into fluid energy
54% efficiency

This result is quite consistent with a 70% efficient pump coupled to an 80% efficient motor and pretty much in line with what I was expecting.

For the axial flow pump

1500 GPH for pressure energy calculation
2000 GPH for kinetic energy calculation
1 ft of head at pump discharge
2 inch diameter opening at pump discharge
50 watts power consumed


1.1 watts power going to kinetic energy
4.7 watts powergoing to pressure energy component of total energy
5.8 watts total energy
11.6% efficiency

This cannot represent reality. Either the head pressure is under estimated or the consumed power is over estimated. If you are estimating the consumed power based on the motor rating for the axial flow pump and motor combo, that estimate is probably too high. Many of the higher end pumps will specify motors rated to handle the worst case operating conditions. In the case of an axial flow pump, that would be when something is restricting the discharge. In that case, the power draw will increase dramatically. A well made axial flow pump should provide 85% plus efficiency. But I see nothing wrong with the calculations you have made. (Hopefully, I did all the unit coversions correctly as well!)

The exercise raises some interesting observations.

1) If one wants high bulk flow at low power consumption a very low head pump and low velocity pump should be chosen

2) Because KE = 1/2 mv^2 the power consumed to create kinetic energy in the water increases rapidly as the velcity increases

3) Since the volute (casing) on a centrifugal pump largely converts kinetic energy to pressure inergy inside the volute, enlarging the discharge nozzle should also be a modification whcih would improve powerhead operation for submerged aquarium uses. (But an oversized volute is not a good idea as too much recirculation internal to the volute can occur)

However, it remains difficult to compare the overall flow induced in an aquarium when trying to select between the two pump types since we don't have a good way to determine how much lower velocity "bulk flow" is created in the tank when the higher velocity flow from the power head interacts witht he tank water. Clearly, a substantial portion of slow moving water will be accelerated by the faster moving jet of water from the power head, but the effciency at which this happens is unknown to me. My general feeling on the two pump types is still unchanged; I suspect that for the same power input the axial flow pump will be moving 40-50% more water in our aquariums, but this difference would be smaller if both types of pumps were well designed and run at their designed flow rates. The difference will get larger if a very high head centrifugal pump is used since the high velocities it will produce will necessarily mean more energy loss when converting pressure energy to flow energy.

Hope this was clear.

By the way, where is So Cal are you?

Tracy

 

[Anonymous]

hi.
Thanx for checking my calculation. For the first calculation, the KE does not match, because I used diameter instead of radius for calcuation. When I use the right number, mine matched yours (15.3 W).

For the second calcuation, both of us did it correctly. However, the problem is that we need to use the pressure/flow rate matching pair. At 2000 gph, the head pressure is zero. So if we blindly plug in the number, the "pressure" energy will be zero. (I hate limit->0 concept.) So instead, I will redo the calculation with flow rate with head pressure of 5 ft. (Sorry for misleading you with the incorrect flow rate, line chart is difficult to read) It is about the same as restrictly the output with a 3/4 inch orifice (close enough...) So:
1500 gph
5 ft head
3/4 in diameter orifice
50 W power

The KE term is 24.1W, while the "PE" term is 4.7W. Total is 28W, so it is about 56% efficiency. This looks slightly better.

I agree with all your points. How much of the energy is transfered from the fast moving stream to the surrounding water is a function of the relative velocity. The viscosity of water is a well know parameter that also play a role in the energy transfer, but other that this, I don't remember anything else in hydrodynamics.

Most propeller pump moves 100% to 200% more water than a powerhead/pump of similar wattage. Your estimate of 40-50% is low IME. However, I am also skeptical to any propeller pump that claims to move 5X as much water as a similar pump. I bet the manufacture did not do a well engineered measurement, and over-inflated the figure.

FYI, a small propeller pump with 1hp motor (750W) pushes ~350 gpm, and is just about enough for someone to swim against. In reef, the surge can exceed many times the flow rate, and small damsel can swim many times as fast as most swimmers. With head pressure of 12 feet, the flow rate drop to only 100 gpm, after which the motor probably overheat. In contrast, most pressure pumps can handle 12 feet easily, but with much lower flow rate.

I am in Orange County.